Astronomical telescope

A telescope is an optical instrument which is used to see the distant objects. Telescopes are of two types:

    1. Astronomical telescope
2. Galilean telescope or Terrestrial telescope

Astronomical Telescope

An astronomical telescope is an optical instrument which is used to see the magnified image of distant heavenly bodies like stars, planets, satellites and galaxies etc.

The final image formed by an astronomical telescope is always virtual, inverted and magnified.

Principle of Astronomical Telescope

An astronomical telescope works on the principle that when an object to be magnified is placed at a large distance from the objective lens of telescope, a virtual, inverted and magnified image of the object is formed at the least distance of distinct vision from the eye held close to the eye piece.

Construction of Astronomical Telescope

An astronomical telescope consists of two convex lenses : an objective lens O and an eye piece E. the focal length fo of the objective lens of astronomical telescope is large as compared to the focal length fe of the eye piece. And the aperture of objective lens O is large as compared to that of eye piece, so that it can receive more light from the distant object and form a bright image of the distant object. Both the objective lens and the eye piece are fitted at the free ends of two sliding tubes, at a suitable distance from each other.

Working of Astronomical telescope

The ray diagram to show the working of the astronomical telescope is shown in figure. A parallel beam of light from a heavenly body such as stars, planets or satellites fall on the objective lens of the telescope. The objective lens forms a real, inverted and diminished image A’B’ of the heavenly body. This image (A’B’) now acts as an object for the eye piece E, whose position is adjusted so that the image lies between the focus fe’ and the optical centre C₂ of the eye piece. Now the eye piece forms a virtual, inverted and highly magnified image of object at infinity. When the final image of an object is formed at infinity, the telescope is said to be in ‘normal adjustment’.

It should be noted that, the final image of object (such as stars, planets or satellites) formed by an astronomical telescope is always inverted with respect to the object. But it does not matter whether the image formed by an astronomical telescope is inverted or not, as all the heavenly bodies are usually spherical is shape.

Magnifying Power of an Astronomical Telescope

The Magnifying Power of a telescope is given by:

m    =    

Where, fo = Focal length of the objective lens
fe = Focal length of the eye-piece lens

And the length (L) of the tube of telescope is equal to the sum of the focal lengths of the objective lens and the eye piece. Thus,

L = fo + fe

Test Your Understanding and Answer These Questions:

1. Define telescope.
2. Name the types of telescope.
3. What is an astronomical telescope?
4. Explain principle, construction and working of an astronomical telescope with the help of a ray diagram.
5. What is magnification of astronomical telescope?
6. Which lens has small focal length in astronomical telescope?
7. How many lenses are used in a telescope?
8. Which instrument is used to see heavenly bodies?